class_docstr: Main coreference resolution algorithm
docstr: Merge two clusters together
docstr_tok: ["Merge","two","clusters","together"]
code: def _merge_coreference_clusters(self, ant_idx, mention_idx):
      """
      Merge two clusters together
      """
      if (
          self.mention_to_cluster[ant_idx]
          == self.mention_to_cluster[mention_idx]
      ):
          return
  
      remove_id = self.mention_to_cluster[ant_idx]
      keep_id = self.mention_to_cluster[mention_idx]
      for idx in self.clusters[remove_id]:
          self.mention_to_cluster[idx] = keep_id
          self.clusters[keep_id].append(idx)
  
      del self.clusters[remove_id]
  
imports: import os
  import spacy
  import numpy
  from neuralcoref.train.utils import PACKAGE_DIRECTORY, SIZE_SINGLE_IN
  from neuralcoref.train.compat import unicode_
  from neuralcoref.train.document import Document, MENTION_TYPE, NO_COREF_LIST
label_desc: Use image node_id 3 for calling the Coref obj's underlying member method code with example usage: obj._merge_coreference_clusters(ant_idx, mention_idx) without return types